I have this fetish as well, I've also had this happen to me. I think if this happens, try to flush yourself out with an enema (Preferably a warm one).
I have never heard of this before. My question is why? Where or what is the pleasure sensation?
Please encourage your wife to be super careful, and if the pain has not yet subsided, to see a doctor just in case she tore her intestine with too much pressure. This seems like a fetish that would not feel very sexy, but if it feels sexy to her, the main thing is that she should not hurt herself or tear anything.
I, myself, have this fetish and have been practicing belly inflation for over 14 years now. It's important that she goes slow and does not try to overdo it. I have only had one time where i was worried about anything adverse happening. I would just talk to her about it and make sure she is doing this in a safe way. Mostly for when she starts to feel pain, she needs to stop until she can be sure if the pain is just a cramp or if it is too much pressure.
Does not sound safe at all.